The present invention relates to biasable system films for use in an electrostatographic, including digital, apparatus. The biasable system films may be useful as bias charging films, sheets, belts or the like, or bias transfer films, sheets, belts, or the like. The biasable system films comprise a polyimide layer filled with or containing a conductive filler, preferably a fluorinated carbon filler. The present invention provides biasable system films with superior electrical and mechanical properties, including controlled conductivity in a desired resistivity range, and increased mechanical strength, durometer, tensile strength, elongation and toughness. Further, in embodiments, the films also exhibit excellent properties such as statistical insensitivity of conductivity to changes in temperature and humidity, intense continuous corona exposure, corrosive environments, solvent treatment, running time or cycling to high electric fields and back. Also, in embodiments, the layers permit a decrease in contamination of other xerographic components such as photoconductors.
Generally, the process of electrostatographic copying is initiated by exposing a light image of an original document onto a substantially uniformly charged photoreceptive member. Exposing the charged photoreceptive member to a light image discharges a photoconductive surface thereon in areas corresponding to non-image areas in the original document while maintaining the charge in image areas, thereby creating an electrostatic latent image of the original document on the photoreceptive member. This latent image is subsequently developed into a visible image by depositing charged developing material such as toner onto the photoreceptive member such that the developing material is attracted to the charged image areas on the photoconductive surface. Thereafter, the developing material, and more specifically toner, is transferred from the photoreceptive member to a copy sheet or to some other image support substrate to create an image which may be permanently affixed to the image support substrate, thereby providing an electrophotographic reproduction of the original document. In a final step in the process, the photoconductive surface of the photoreceptive member is cleaned to remove any residual developing material which may be remaining on the surface thereof in preparation for successive imaging cycles.
Biasable members include both bias transfer members and bias charging members. Toner material can be transferred from a first image support surface (i.e., a photoreceptor) into attachment with a second image support substrate (i.e., a copy sheet) under the influence of electrostatic force fields generated by an electrically biased member, wherein charge is deposited on the second image support substrate by, for example, a bias transfer member or by spraying the charge on the back of the substrate.
An important aspect of the transfer process focuses on maintaining the same pattern and intensity of electrostatic fields as on the original latent electrostatic image being reproduced to induce transfer without causing scattering or smearing of the developer material. This important and difficult criterion is satisfied by careful control of the electrostatic fields, which, by necessity, should be high enough to effect toner transfer while being low enough to not cause arcing or excessive ionization at undesired locations. These electrical disturbances can create copy or print defects by inhibiting toner transfer or by inducing uncontrolled transfer which can easily cause scattering or smearing of the development materials.
Contact charging or bias charging members function by applying a voltage to the charge-receiving member (photoconductive member). Such bias charging members require a resistivity of the entire charging member within a desired range. Specifically, materials with too low resistivities will cause shorting and/or unacceptably high current flow to the photoconductor. Materials with too high resistivities will require unacceptably high voltages. Other problems which can result if the resistivity is not within the required range include low charging potential and non-uniform charging, which can result in poor image quality.
Therefore, it is important in biasable members, that the resistivity be tailored to a desired range and that the resistivity remain within this desired range. Accordingly, it is desirable that the resistivity be unaffected or virtually unaffected to changes in temperature, relative humidity, running time, and leaching out of contamination to photoconductors.
Attempts at maintaining an acceptable transfer field with regard to bias transfer members, have included adding ionic additives to elastomer layers of bias transfer members in an attempt to control the resistivity. U.S. Pat. Nos. 3,959,573 and 3,959,574 both to Seanor et al. describe adding additives such as a quaternary ammonium compound to hydrophobic and hydrophilic elastomeric polyurethane layers, respectively, in order to control the changes in resistivity due to changes in relative humidity. Similarly, U.S. Pat. Nos. 5,286,570, 2,259,990, 2,586,566 and 2,259,989, all to Schlueter, Jr. et al., describe the addition of an asymmetric ionic quaternary ammonium salt to a polyurethane elastomer to extend the useful electrical life of the polyurethane elastomers.
Attempts at controlling resistivity with regard to bias charging members have included adding ionic additives to elastomer layers. European Patent Application 0 596 477 A2, discloses a charging member comprising at least an elastic layer comprising epichlorohydrin rubber and a surface layer disposed thereon, the surface layer comprising at least a semiconductive resin and an insulating metal oxide contained in the semiconductive resin.
However, there are problems associated with the use of such additives. In particular, undissolved particles frequently bloom or migrate to the surface of the polymer and cause an imperfection in the polymer. This leads to nonuniform resistivity, which in turn, causes poor antistatic properties and poor mechanical strength. The ionic additives on the surface may interfere with toner release. Furthermore, bubbles may appear in the conductive polymer, some of which can only be seen with the aid of a microscope, others of which are large enough to be observed with the naked eye. These bubbles provide the same kind of difficulty as the undissolved particles in the polymer, namely poor or nonuniform electrical properties and poor mechanical properties.
In addition, the ionic additives themselves are sensitive to changes in temperature, humidity, and operating time. These sensitivities often limit the resistivity range. For example, the resistivity usually decreases by up to two orders of magnitude or more as the humidity increases from 20 percent to 80 percent relative humidity. This effect limits the operational or process latitude.
Moreover, ion transfer can also occur in these systems. The transfer of ions leads to charge exchanges and insufficient transfers, which in turn causes low image resolution and image deterioration, thereby adversely affecting the copy quality. In color systems, additional adverse results are color shifting and color deterioration. Ion transfer also increases the resistivity of the polymer member after repetitive use. This can limit the process and operational latitude and eventually the ion-filled polymer member will be unusable.
Conductive particulate fillers, such as carbon blacks, have also been used in an attempt to control the resistivity.
U.S. Pat. No. 5,112,708 to Okunuki et al., discloses a charging member comprising a surface layer formed of N-alkoxymethylated nylon which may be filled with fluorinated carbon.
U.S. Pat. No. 5,000,875 to Kolouch discloses tetrafluoroethylene copolymer compositions containing conductive carbon black or graphite fibers to increase conductivity when the tetrafluoroethylene copolymer has been treated with a fluorinating agent.
Carbon black particles can impart specific adverse effects. Such carbon dispersions are difficult to prepare due to carbon gelling, and the resulting layers may deform due to gelatin formation. In addition, the required tolerance in the filler loading to achieve the required range of resistivity is extremely narrow. This, along with the large "batch to batch" variation, leads to the need for extremely tight resistivity control. In addition, carbon filled polymer surfaces have typically had very poor dielectric strength and sometimes significant resistivity dependence on applied fields. This leads to a compromise in the choice of centerline resistivity due to the variability in the electrical properties, which in turn, ultimately leads to a compromise in performance.
U.S. Pat. No. 5,397,863 discloses film capacitors using polyimide materials and fluorinated carbons.
U.S. Pat. No. 5,556,899 discloses adding fluorinated carbon to polyimide materials to effect a change in the dielectric constant and the coefficient of thermal expansion of the polyimide for use in electronic packaging.
U.S. Pat. No. 5,571,852 discloses use of fluorinated carbon in polyimide materials for electrical conductor patterns.
U.S. Pat. No. 5,591,285 discloses adding fluorinated carbon to fluoropolymers and exposing the material to ultraviolet radiation for electronic packaging applications.
There exists a specific need for bias system films which allow for a stable conductivity in the desired resistivity range without the problems associated with ionic additives and carbon additives.